1. Field of the Invention
The present invention relates generally to an image signal compressor for compressing the voltage level of an image signal in excess of a predetermined compression voltage level, and more particularly to a control circuit or a controller incorporated in the image signal compressor connected to an image pickup tube or solid-state image pickup elements.
2. Description of the Prior Art
The background of the present invention will be explained with respect to its application to an image signal compressor connected to an image pickup tube.
The image signal compressor is a kind of converter, through which the voltage level of a signal in excess of a predetermined level is reduced or compressed within a predetermined voltage range at signal transmission terminals.
In the compressor, a knee level is usually determined at which the voltage level of a signal begins to be compressed. Further, a limiter is additionally connected to the output terminal of the compressor for further securely limit the peak voltage level of the compressed signal.
In the case where the signal to be compressed varies in saw tooth waveform fashion for instance, the signal is derived from the compressor as it is without compression as far as the signal voltage level is below the knee level, but compressed by the compressor at a predetermined compression ratio when the signal voltage level is beyond the knee level. The voltage level of the compressed signal increases with increasing time elapsed until the original signal voltage level reaches its peak value. Therefore, if the peak voltage value of the signal to be compressed is constant, the peak value of the compressed signal may be constant without being further limited or clipped by the limiter. However, if the peak value of the signal to be compressed varies to still a higher value, there exists a problem in that the peak voltage value of the compressed signal may rise beyond the limiter level, so that the compressed signal may be clipped by the limiter without being compressed normally. This is because the knee level is fixedly predetermined in the compressor and thereby the image signal having a high peak voltage value is compressed beyond a predetermined limiter level.
The circuit configuration and its operation of an example of prior-art image signal compressors will be described in further detail hereinafter with reference to the attached drawings under DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT.